Regulated capacitive loading and gain control of a crystal oscillator during startup and steady state operation

Abstract

An oscillator circuit and system are provided having a peak detector that can determine a peak voltage value from the oscillator. The peak voltage value can then be compared against a predetermined voltage value by a controller coupled to the peak detector. The comparison value is then used to change a bias signal if the peak voltage value is dissimilar from the predetermined voltage value. A variable capacitor or varactor can be formed from a transistor and is coupled to the oscillator for receiving the bias signal upon a varactor bias node. The bias signal is used to regulate the capacitance within the varactor as applied to the oscillator nodes. Another controller can also be coupled to the peak detector to produce a second bias signal if the peak voltage is dissimilar from a second predetermined voltage value. The second bias signal can then be forwarded into an amplifier having a variable gain to regulate the gain applied to the oscillator. The combination of a varactor and variable gain amplifier regulate the negative resistance applied to the resonating circuit during startup and steady state operations to ensure a relatively fast startup, and to maintain optimal loading and accurate steady state amplitude after startup has completed.

1. A system, comprising:
 an oscillator; 
 a peak detector coupled to determine a peak voltage value from the oscillator; 
 a controller coupled to the peak detector for comparing the peak voltage value against a predetermined voltage value; 
 a varactor coupled to the oscillator, wherein the controller is configured to change a bias signal applied to a bias node of the varactor to regulate a capacitance applied to the oscillator when the peak detector indicates that the peak voltage value is dissimilar in absolute magnitude than the predetermined voltage value, and wherein the controller is further configured to regulate the capacitance across a range of load capacitances using the bias signal; 
 a second controller coupled to the peak detector for comparing the peak voltage value against a second predetermined voltage value, and to change a second bias signal if the peak voltage is dissimilar in absolute magnitude than the second predetermined value; and 
 an amplifier coupled to the oscillator and having an amplifier bias node upon which the second bias signal is forwarded to regulate a gain applied to the oscillator. 
 
     
     
       2. The system as recited in  claim 1 , wherein the varactor comprises a metal oxide semiconductor transistor having a source and drain region coupled together. 
     
     
       3. The system as recited in  claim 1 , further comprising:
 an input node; 
 an attenuator coupled to the input node; and a low-pass filter coupled between the attenuator and the bias node. 
 
     
     
       4. The system as recited in  claim 3 , wherein the input node is adapted to receive a control voltage that, when forwarded through the attenuator and the low-pass filter, provides a capacitor control voltage to the varactor bias node. 
     
     
       5. The system as recited in  claim 1 , wherein the peak detector comprises:
 a holding capacitor coupled to store the peak voltage value; 
 a comparator adapted to compare the stored peak voltage against a sinusoidal signal; and 
 a current source coupled to and controlled by an output of the comparator for changing voltage on the holding capacitor if the sinusoidal signal is dissimilar in amplitude than the peak voltage value. 
 
     
     
       6. The system as recited in  claim 1 , wherein the controller and the second controller each comprises:
 a differential amplifier adapted to amplify a difference between the peak voltage value and the predetermined voltage value. 
 
     
     
       7. The system as recited in  claim 6 , wherein the controller further comprises a transistor coupled to an output of the differential amplifier to forward a first amount of current in the bias signal if the peak voltage value is less than the predetermined voltage value, and to forward a second amount of current greater than the first amount of current in the bias signal if the peak voltage value is greater than the predetermined voltage value. 
     
     
       8. The system as recited in  claim 7 , wherein the first amount of current applied to the varactor decreases the capacitance of the varactor relative to the second amount of current applied to the varactor. 
     
     
       9. The system as recited in  claim 7 , wherein the first amount of current is applied during start up operation of the oscillator and the second amount of current is applied after start up operation of the oscillator. 
     
     
       10. The system as recited in  claim 7 , wherein a third amount of current between the first and second amounts of current is forwarded when the peak voltage value is equal to the predetermined voltage value. 
     
     
       11. The system as recited in  claim 10 , wherein the third amount of current is applied during steady state operation of the oscillator. 
     
     
       12. An oscillator, comprising:
 a pair of nodes; 
 a piezoelectric material coupled between the pair of nodes; 
 an amplifier coupled between the pair of nodes for producing a sinusoidal signal upon the pair of nodes during initial startup operation of the oscillator and during subsequent steady state operation of the oscillator; 
 a peak detector coupled to at least one node of the pair of nodes for determining a peak voltage value of the sinusoidal signal; 
 a controller coupled to compare the peak voltage value with a predetermined voltage value and to apply a first capacitive load to the pair of nodes during initial startup if the peak voltage value is less than the predetermined voltage value and to apply a second capacitive load to the pair of nodes greater than the first capacitive load during steady state if the peak voltage value is equal to or greater than the predetermined voltage value; and, 
 a second controller coupled to the peak detector for comparing the peak voltage value against a second predetermined voltage value, and to change a gain of the amplifier when the peak voltage is dissimilar in absolute magnitude than the second predetermined value. 
 
     
     
       13. The oscillator as recited in  claim 12 , further comprising a varactor that includes a transistor with a mutually coupled source and drain region and a gate region coupled to receive a bias signal whose magnitude is dependent upon a comparison of the peak voltage value and the predetermined voltage value. 
     
     
       14. The oscillator as recited in  claim 12 , wherein the bias signal is a current applied to a node which forms a voltage upon the gate for placing the transistor in a depletion mode if the current is of a first current value and for placing the transistor in an inversion mode if the current is of a second current value greater than the first current value. 
     
     
       15. The oscillator as recited in  claim 12 , wherein the change of gain of the amplifier decreases if the peak voltage is greater in absolute magnitude than the second predetermined value. 
     
     
       16. The oscillator as recited in  claim 15 , wherein the first capacitive load is applied to the pair of nodes, followed by the second capacitive load applied to the nodes, followed by the decrease in gain of the amplifier. 
     
     
       17. A method for regulating an oscillator output, comprising:
 detecting a peak voltage output from the oscillator; comparing the peak voltage to a reference voltage; 
 decreasing the capacitive loading on the oscillator if the peak voltage is less than the reference voltage during start up of the oscillator; 
 optionally increasing the capacitive loading on the oscillator if the peak voltage is greater than the reference voltage subsequent to start up of the oscillator; and, 
 changing amplifier gain when the peak voltage is dissimilar in absolute magnitude relative to another reference voltage. 
 
     
     
       18. The method as recited in  claim 17 , further comprising decreasing gain from the oscillator subsequent to said increasing.